This invention relates to a melt polymerization process for preparing a polyarylate wherein a diester of a dihydric phenol is condensed with an aromatic dicarboxylic acid in the presence of a catalytically effective amount of a catalyst comprising an imidazole and, optionally, as a co-catalyst, an alkali or alkaline earth metal salt of an acid of pKa&gt;3. Such a process allows the condensation of proceed at a lower initial temperature and has a faster melt polymerization reaction rate when compared to similar processes which do not employ such a catalyst or catalyst system; and the polyarylate so produced has improved color.
Polyarylates have many superior properties which make them useful as engineering thermoplastics, including good mechanical properties, high heat distortion temperature, good fire retardancy, and good solvent resistance. They also have good processability which allows them to be molded into articles.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process a dihydric phenol is converted to the diester derivative by reacting it with an acid anhydride and then the diester derivative is reacted with an aromatic dicarboxylic acid(s) to form the polyarylate. For example, when the reaction product of bisphenol A and terephthalic acid is prepared by the diacetate process using acetic anhydride, the following reaction sequence takes place: ##STR1##
The condensation of the diester with the diacid according to the invention is accomplished by melt polymerization, thereby avoiding the use of costly solvents, minimizing environmental pollution, and reducing reactor volume. In such a polymerization process, however, long reaction times at relatively high temperatures, especially when terephthalic acid is one of the reactants, results in degradation of the reaction mixture components thereby generating an undesirable color, varying from dark yellow to brown, in the polymer product.
It has now been found that reaction times can be shortened and the intensity of the color lessened by catalyzing the melt polymerization reaction with a catalyst system comprising an imidazole and, optionally, as a co-catalyst, an alkali or alkaline earth salt of an acid of pKa&gt;3. Although the mechanism of the catalytic action of the imidazole is not completely understood, it is believed that, when terephthalic acid is present, its function is two-fold, aiding in the dissolution of the very insoluble terephthalic acid by forming a more soluble salt, and acting as a conventional catalyst.
The acid anhydride suitable for use herein is derived from a monocarboxylic acid containing from 1 to 7 carbon atoms. The preferred acid anhydride is acetic anhydride, which is commercially available and has a low reflux temperature.
Any suitable dihydric phenol or admixtures thereof may be used herein. The preferred dihydric phenols are resorcinol; hydroquinone; diphenol; dihydroxynaphthalene; bis(4-hydroxyphenyl)methane (BPF or bisphenol F); 1,1- and 1,2-bis(4-hydroxyphenyl)ethane (BPE or bisphenol E); bis(4-hydroxyphenyl)sulfide; 2,2-bis(4-hydroxyphenyl)propane (BPA or bisphenol A); bis(4-hydroxyphenyl)oxide; and bis(4-hydroxyphenyl)sulfone. Among the especially preferred dihydric phenols are the bisphenols, BPF, BPE, and BPA. Contemplated as equivalents are substituted dihydric phenols, such as alkyl and halogen substituted derivatives, provided, however, that these substituents do not adversely affect the properties of the polymer or significantly diminish their ability to melt polymerize.
Any suitable aromatic dicarboxylic acid or admixtures thereof may be used herein. The preferred acids are terephthalic acid; isophthalic acid; bibenzoic acid; naphthalene dicarboxylic acid; bis(4-carboxyphenyl)methane; 1,2-bis(4-carboxyphenyl)ethane; 2,2-bis(4-carboxyphenyl)propane; bis(4-carboxyphenyl)oxide; bis(4-carboxyphenyl)sulfide; and bis(4-carboxyphenyl)sulfone. More preferred are terephthalic and isophthalic acid, and most preferred is terephthalic acid. Improvement in reaction rate is most notable when the acid comprises at least 50 mol percent terephthalic acid. Contemplated as equivalents are substituted aromatic dicarboxylic acids, such as alkyl and halogen substituted derivatives, provided, however, that these substituents do not adversely affect the properties of the polymer or significantly diminish their ability to melt polymerize.
Any suitable imidazole catalyst which yields an increase in melt polymerization reaction rate may be used herein. Normally such imidazoles will have a pKa&gt;5. Based on improved reaction rate, the preferred imidazole catalysts are imidazole, 1-methylimidazole, 1-ethylimidazole, 2-ethyl-4-methyl imidazole, and benzimidazole. Especially preferred are imidazole and 1-methylimidazole, and most preferred, based on a combination of improved reaction rate and less intense color in the product, is 1-methylimidazole.
Suitable co-catalysts are any alkali or alkaline earth metal salt of any acid of pKa&gt;3. Based on less intense color in the product, the preferred alkali or alkaline earth metal salts of acids of pKa&gt;3 are selected from the group consisting of sodium, potassium, or lithium acetates, carbonates, benzoates, formates, bicarbonates, phosphates and monohydrogen phosphates. Especially preferred are lithium, sodium and potassium carbonates, acetates, phosphates, and monohydrogen phosphates; and most preferred are potassium carbonate, acetate, and phosphate.
Formation of the diester by reaction of the dihydric phenol with the acid anhydride may be accomplished under conventional esterification conditions. This reaction may take place in the presence or absence of a solvent. Optionally, a conventional esterification catalyst may be added to the reaction mixture. When the diester is formed in the presence of a solvent, the product is isolated by conventional techniques prior to the melt polymerization process of this invention.
The preparation of the polyarylate from the diester and the aromatic dicarboxylic acid is carried out by a melt polymerization process. Typically, approximately equimolar amounts, mole ratio=0.95 to 1.05, diester and acid are charged to the reactor. An effective amount of an imidazole catalyst of pKa&gt;5, advantageously in the range of 1.times.10.sup.-3 mole to about 3.times.10.sup.-2 moles, preferably, between 3.times.10.sup.-3 and 2.times.10.sup.-2 moles, per mole of diester and, optionally, an effective amount of an alkali or alkaline earth metal salt of an acid of pKa&gt;3, advantageously in the range of about 2.times.10.sup.-6 mole to about 5.times.10.sup.-3 mole per mole of diester, preferably between 5.times.10.sup.-6 and 4.times.10.sup.-3 mole, are also charged. The reaction mixture is then heated and, as the mixture is heated, the monocarboxylic acid of anhydride, which forms is removed advantageously at a temperature in the range of about 230.degree. C. to about 300.degree. C. A vacuum is normally applied (final pressure.about.0.2-5 Torr) to facilitate removal of essentially all the monocarboxylic acid.
Formation of the diester and the polyarylate may be accomplished in separate reaction vessels. In such a case, the acid formed and the excess acid anhydride from the diester reaction in the first reactor may be removed by known techniques such as vacuum distillation or by chemical reaction. The diester is then reacted in the second reactor with the aromatic dicarboxylic acid in the presence of the imidazole catalyst or a catalyst system additionally comprising, as a co-catalyst, an alkali or alkaline earth metal salt of an acid of pKa&gt;3 to form the polyarylate by melt polymerization.
Alternatively, both reactions may be performed in one reactor by first forming the diester at a temperature between 90.degree. C. and 140.degree. C., then removing the acid and excess acid anhydride as before, and finally forming the polyarylate at a temperature between 230.degree. C. and 300.degree. C. Thus all the reactants may be included in the initial reaction mixture and the reaction steps are controlled by the temperature.
Formation of the diester is advantageously carried out at a temperature sufficient to maintain reflux, usually between 130.degree. and 140.degree. C. when acetic anhydride is the esterification agent, and usually requires between 0.5 and 1.5 hours. Generally, the acid anhydride is used in excess, advantageously in the range of about 5 to 20 mole % excess. Formation of the polyarylate is effected at a temperature which maintains the reaction mixture in a molten state, advantageously in the range of 230 and 325.degree. C., and preferably between 240.degree. C. and 290.degree. C. This reaction is preferably carried out in an inert atmosphere and at a reduced pressure, preferably less than 5 Torr., to facilitate removal of the monocarboxylic acid reaction product and drive the polycondensation to completion.
The process may be used to form a polyarylate with an inherent viscosity between 0.1 and 2.0 dl/g, preferably between 0.5 and 1.0 dl/g. The inherent viscosity is determined at 25.degree. C. in a 60/40 weight mixture of phenol and sym-tetrachloroethane at a concentration of 0.5 gram per deciliter.
The melt polymerization process may be accomplished batchwise or continuously and by using any apparatus known in the art.
Stabilizers, colorants, processing aids, fillers and pigments may be added to the reaction mixture(s) provided that they do not react with it or with the products.